@ArtcoInc
If I understand well, probably you are missing GET forms with POST forms. All modul use parameters in href link. tep_draw_hidden_field() not means POST parameters in these cases because hidden field transformed in HTTP header as GET parameters.
If
search_in_description = 1
than this means search results from product_description while 0 not search in products_description.
Store search bar call direct ajax POST method header link with href="" parameters instead use hidden field in a search GET form.

@ArtcoInc
Its not possible in near furure. I am working on v2.4 core but there is hard coded BS3. Language search functionality is more close to v2.4 than v2.3 codebase. When v2.4 core will be totaly separated into 2 parts (core and template) then will be a chance to use different template systems. We are in a sack street at this moment.

Well mostly Preparing [Paypal Standard] status means that the checkout was canceled by the customer. Do you have incomes in PP account? If yes then may be call back url was missed or orders_id destroyed in orders_status_history data table. Check you PP incomes and orders_status_history table if are there any rows with null orders_id. This issue could be happen by admin side too when admin edit orders. This could be a session regeneration problem at admin side.

I dont understand this function. Why do you use it? If you want to disable all cookies the HTML post method enable it by the new managment mode. https://github.com/Gergely/oscommerce_ce/blob/a8838c9532dc9227a7f694df935badf2ab8d78ca/includes/classes/osc_template.php#L204-L211

@azpro what is php version?
Strict cookies could be extend in modal template with a foreach function.
What opt-out function is? @wHiTeHaT
I dont see the installed/non installed on/off modules problem in your code example.
oscTemplate class could handle all modules, not only the ht_ modules. For example google tag manager code implemented under the <body> tag and core be able to use content module with php cookie.
I dont mind that you are extend or edit this code example. This could be a real function of github. I dislike programing in posts.

@azpro
Thank you for testing. Yes with this solution the core will be slover. (more function more time) I can not do more service with timeless running.
osc_template is the most closest solution at this moment regards to template modules. If we use separated class then we have to reopen all installed modules again so this was non-logic step which was skipped. I tried to do my best with speed optimatization so you can see this code change now.
$_expiration declaration could be cause troubles but I dont know what system do you use. Could you post here the system report result?
System report: https://yourdomain/youradmin/server_info.php?action=export

Dinamyc document managments has a risk that somebody can owerwrite it which is danger. If shop GDPR rules will be changed then you have to ensure the static safety content day by day.
And by this way you can do notifications if need.
PDF format is developed for it.